1. Field of the Invention
The present invention relates to finishing operations for products made from magnesium alloys, and particularly to non-chromate chemical treatments which form a protective film to protect the product from surface corrosion.
2. Description of the Prior Art
Magnesium alloys have many advantages, such as low density, high thermal conductivity and high conductivity of the surface thereof. Because magnesium alloys can comply with the trend forward lighter, thinner, shorter, and smaller fittings having better heat dissipation and improved EMI shielding characteristics for electronic apparatus, magnesium alloys is replacing plastics and become the newest popular material of mobile computer, consumer and mobile communication electronic products. However, magnesium alloys are vulnerable to oxidation in air, so the surfaces of the alloys need a protective film for inhibiting corrosion.
Chromate is normally used to chemically modify surfaces of products made from magnesium alloys. Such as the chemical treatment operations for magnesium alloys proposed by Dow Chemical Company and U.S. Pat. No. 4,676,842 disclose the use of chromate to chemically modify metallic materials, which forms a compact protective film inhibiting the corrosion on surfaces of magnesium alloys. Chromic ions (Cr.sup.3+) in the film and the acid pickle are poisonous and dangerous pollutant to the environment and result in recycling difficulties of magnesium alloys and increased cost of manufacture and waste handling. Since chromic ions (Cr.sup.3+) are to be banned from use in the near future, a reliable operation without using chromate is desired.